It should be pointed out that you could show one person a few math formulas and they’d quickly grok higher mathematics with minimal prompting. You could laboriously walk another person through basic algebra, over a course of several months, and discover that they’re mostly incapable of ever learning it.

And yet, math is a subject that’s probably the most well-suited to teaching on a platform like YouTube - allowing one to show all sorts of visualizations that help to explain a topic that’s entirely cerebral.

So when asking this, you also have to take into account the person.

Like, I could cavalierly say that anything with “hands-on” components like being a car mechanic would be unteachable - even given all the material on YouTube and careful organization to put it all into a linear framework - since you need to actually get out there and practice on something. But that ignores that some people would go and get some practice cars, try to apply the information that they got from the videos, rewatch segments, and search for more videos that helped to fill out the details as they encountered gaps in the training sessions.

And, again, others would watch through the videos, vaguely understand what they were watching in that moment, take no steps to practice any practical application, think almost nothing about what they were seeing nor imagine follow-up questions, and then eject 99.9% of all of the information that they watched through in about 5 minutes of completing the last lecture.

My guess would be that, for the right person, a significant majority of jobs are learnable through YouTube. The exception might be things that are both very technical and very low-applicability - like submarine piloting. You could probably learn most of the high-level technical knowledge, but there’s just not enough people doing the job for there to be enough material produced that you could learn the on-the-job elements that are necessary to go straight from arm-chair to practical application. You’d still need some on-the-job training.

But, for you (which I say, not directed towards any particular “you”), it may be that almost nothing is learnable, purely through YouTube, because you aren’t able to self-motivate, to self-organize, etc. to be able to engage in self-learning.